Process of tanning with salt of ampho-



United States ?atent PROCESS OF TANNING WITH SALT OF AMPHO- TERIC CONDENSATION PRODUCT N Drawing. Application June 25, 1952, Serial No. 295,558

Claims priority, application Germany June 30, 1951 4 Claims. (Cl. 8-94.24)

The present invention relates to a new tanning process.

Tanning with vegetable tannins is based on the action of high molecular weight organic compounds of phenolic character upon animal hides. These compounds owe their reactivity and water-solubility to the abundance of phenolic hydroxyl groups they contain. During the past decades synthetic organic tanning agents which, in most cases, contain besides phenolic hydroxyl groups the water-solubilizing sulphonic acid group in the molecule, have been adopted in tanneries to an increasing degree; such synthetic organic tanning agents are mostly applied in mixture with vegetable tannins. The water-solubility of the vegetable and synthetic tanning agents is caused by the presence of substances of acid nature. The compounds are anionic. Furthermore, it is common to both the vegetable and synthetic compounds that the intensity of tanning increases with increasing acidity of the tanning agent solutions.

in contrast to the foregoing, the basic chromium sulphates, i. e. the tanning agents employed in the technically important chrome tanning, show, in the acid range, poor tanning properties and the intensity of tanning increases when the pH-value of the tanning solutions approaches the neutral point. A tanning process utilizing organic substances, analogous to chrome tanning, has not been known up to the present.

We have found that a tanning process analogous to chrome tanning can be carried out with organic compounds comprising amphoteric aromatic compounds containing, as cationic group, the amino group and, as anionic group, the phenolic hydroxyl group in excess over the cationic groups. Compounds which are suitable according to the invention are obtained for instance according to the processes of our copending application Ser. No. 241,693 filed August 13, 1951. In that application we have described new, water-soluble condensation products obtained by treating at temperatures up to about 100 C. mixtures containing polyhydric phenols, salts of nitrogen-containing compounds of the general formula wherein R stands for a radical selected from the group consisting of hydrogen, alkyl, cycloalkyl, aryl and aralkyl, and R and R stand for radicals selected from the group consisting of hydrogen and alkyl, and saturated lower aliphatic aldehydes. These compounds show the following properties: they are water-soluble in the form of their chlorhydrates in a more or less acid reaction; they are completely precipitated by neutralizing their solution with alkalies and dissolved again as phenolates when excess alkali is added; hence it follows that the compounds are amphoteric in character; the acid solutions are completely precipitated by the solutions of substantially or exclusively anionic organic compounds such ice as vegetable or synthetic tanning agents and lignin sulphonic acid. The new water-soluble condensation products used in the invention may be obtained in accordance with the method described above, i. e. by treatment at temperatures up to about 100 C., of mixtures containing polyhydric phenols, salts of nitrogen-containing compounds and saturated lower aliphatic aldehydes. If desired, however, as is specifically shown in the aboveidentified copending application, the amine may be used in the form of the free base, then salified after the condensation to form the water-soluble product utilized.

The good tanning properties of the aforesaid substances depend in the same way as with the known organic tanning agents, on the presence of a sufiicient number of active groups, i. e. phenolic hydroxyl groups. It is therefore a surprising feature of the invention that tanning by means of the said substances is the more intense and the adsorption of tanning agent the higher, the more the acidity of the tanning agent solution approaches the weakly acid or even the neutral point, which tendency can be promoted by neutralizing the tanning agent solutions during the tanning process as in chrome tanning by addition of controlled quantities of alkali.

The process of the present invention may be carried out by adding the tanning agents to the pelts in portions. The tanning agent solution is neutralized thereafter by gradual addition of ammonia or an alkali until the de sired exhaustion of the tanning agent solution has taken place.

In carrying out the process as described above fully tanned leather of excellent properties is obtained. In dyeing leather tanned in this way the dyestufi is adsorbed to an extraordinarily high degree.

Mixtures of the abovesaid products with vegetable or synthetic organic tanning agents, or with lingin sulphonic acids, as already mentioned above, cannot be employed for tanning because of the tendency of said tanning agents to precipitate one another.

It is possible however to adapt the new tanning process as is common practice in other cases to the so-called combination tannage, i. e. the new process may be applied for pretanning or retanning hides which are to be or have been subjected to another tanning process for instance with vegetable, synthetic, or mineral tanning agents, formaldehyde, paraflin sulphochlorides and fish oil.

In the combination tannage with vegetable and synthetic tanning agents and lignin sulphonic acids, additional valuable results are obtained by the mutual influence of the said tanning agents on the one hand and the amphoteric tanning agents on the other hand, which leads to particularly water-resistant leather of a highly reduced content of substances which are capable of being washed out. By treating hides for instance with lignin sulphonic acids a fast fixation is attained only when the reaction is carried out in an acid range (pH below 4). The hides thus treated however yield no commercial leather on drying. By retanning such hides however with the amphoteric tanning agents a very high adsorption of tanning agent takes place though the tanning agent solutions generally adjust themselves during retanning to a substantially higher pH value. A further increase in the adsorption of tanning agent and filling eifect of the leather thus obtained is achieved by gradual neutralization of the tanning agent solution. Owing to the action of the amphoteric tanning agent the lignin sulphonic acid is excellently fixed to the leather despite the faintly acid reaction of the final tanning agent solutions.

In the usual retanning of neats leather the fully tanned, pressed leather is milled in a hot-air drum with the tanfling agent extract and, subsequently, with a fixing agent which is capable of precipitating the tanning agent such as lue and casein. In most cases, a substance of acid nature such as oxalic acid is added to improve fixation and colour of the leather. By replacing in this method the fixing agent and furthermore the retanning extract by the said amphoteric tanning agents, besides a fast fixation retanning takes place, which in contrast to the above mentioned usual retanning method, is the stronger the less acid the conditions of the process. A retanning mixture assisted by the water-soluble salts of the amphoteric tanning agents is in any case generally substantially less acid than the usual retanning extract and fixing mixtures. The intensity of retanning by means of the amphoteric tanning agents can further be increased by milling a weakly alkaline reacting agent, such for example as sodium acetate, into the leather during retanning.

In this manner the colour of the leather is simultaneously improved and lightened which according to the prior art method has to be accomplished conversely, i. e., by addition of substances of acid nature. According to the process of the invention the hazard of the durability of the leather being impaired by the presence or concentrated acids is safely avoided. Such leather is especially water-resistant since the amphoteric tanning agents show the combined property of tanning and acting as fixing agent for the tanning agent.

The invention is illustrated by the following examples, the parts being by weight:

Example 1 100 parts of calf pelts are tanned as set forth below in 100 parts of water with a solution of 60 parts of a tanning agent containing 34% of pure tanning agent which is obtained from resorcinol, ammonium chloride and formaldehyde as described in Example 33 of copending application No. 241,693 in 50 parts of water. More specifically, the process may be carried out by adding 102 cc. of formaldehyde solution 30% to a solution of 18 grams of ammonium chloride and 110 grams of resorcinol in 60 cc. of water at room temperature. The temperature of the mixture rises to above 90 C. within a few minutes and it is then heated for an additional 1 /2 hours in a boiling water bath to obtain the desired water-soluble condensation product.

The tanning agent solution is added in 5 equal portions in intervals of 1 hour each. By moving the material about inside a drum for 1 /2 days after addition of the last portion of tanning agent solution the pH value of the solution is 4.4-4.6 and is slowly adjusted thereafter to about 6 by addition of a dilute ammonia solution. any turbidity occurring when ammonia is added disappears after a short time. The leather is moved in the neutralized tanning agent solution for another 16 hours, thereafter shortly rinsed, slightly stuffed and dried. An extraordinarily fully tanned, fine leather of light colour and bark tanned character is thus obtained.

Example 2 100 parts of goat pelts are tanned as described in Example 1 with a tanning agent solution of 40 parts of a tanning agent (content of pure tanning agent 40%) obtained from brenzoel, aniline hydrochloride and formaldehyde according to Example 8 of copending application No. 241,693. Thus, a mixture of 5 cc. of concentrated hydrochloric acid and 5 cc. of Water is added at C. to a solution of 125 grams of brenzoel in 50 cc. of water and 51 cc. of formaldehyde solution 30%. The temperature of the mixture slowly rises to about 80 C. and the mixture is then refluxed for half an hour. The resulting mixture is then neutralized with 3 grams of anhydrous sodium carbonate. To the neutralized mixture 65 grams of aniline hydrochloride and 6.5 cc. of water are added. The temperature of the mixture is cooled to 5-8" C. then 51 cc. of formaldehyde solution 30% is quickly run in with ice-cooling and stirring. A resin separates which dissolves on heating in a water bath at 70 C. This mixture, when heated for an additional half hour in a boiling water bath, yields the Water-soluble condensation product. After moving the hides in the drum for 45 hours the pH value in the tanning agent solution has adjusted to 44.2. By slow addition of a dilute ammonia solution a pH value of 5.55.7 is obtained. After the usual rinsing, stufiing and drying a fully tanned, yellowbrown leather of compact feel is obtained.

The term brenzoel mentioned above denotes a part of the so-called phenolic oils which are recovered from waste liquors formed in the low temperature carbonization process of certain lignites and pit-coals by extraction with organic solvents. From the phenolic oils the monovalent phenols are removed by distillation. The residue is the so-called brenzoel, which contains catechol and homologous substitution products of catechol such as homocatechol, isohomocatechol, ethyl catechol etc., and to a lesser extent also resorcinol and its homologues, as well as remnants of substituted monovalent phenols.

Example 3 100 parts of goat pelt are pretanned at a pH of 3-3.2 in the usual manner with 50 parts of a commercial lignin sulphonic acid product having a content of pure tanning agent of 40%. The flesh-side and thereafter the grainside of the leather thus pretreated are placed on a double quire of filter paper for 5 minutes each and hung in the air for 1 hour. The average thickness of the leather is determined by measuring at various spots. The leather is then retanned with 27 parts of the tanning agent employed in Example 2, which is prepared from brenzoel, aniline hydrochloride and formaldehyde (content of pure tanning agent 40%) in the following manner.

The pretanned leather is treated in 100 parts of water with the solution of the tanning agent in 50 parts of water by adding the tanning agent solution in 5 portions in intervals of 1 hour each. After a running time of about 2-3 days the leather is taken out of the solution (final pH value 4.4), rinsed, stuffed and dried. A light yellowbrown, soft, fine, fully tanned leather of a good feel is obtained which shows an increase in thickness of about 30% as compared with leather (final pH value 3.03.2) which was retanned with another quantity of the commercial lignin sulphonic acid product. The leather tanned with sulphite waste liquor only is very hard, darkbrown coloured and forms cracks on breaking.

Example 4 100 parts of goat pelt are pretanned in the usual man nor with 50 parts of a commercial lignin sulphonic acid product as described in Example 3 (pH value 3.8). The leather is then retanncd in the manner described in Ex ample 3 with parts of the tanning agent mentioned in Example 1, which is obtained from resorcinol, ammonium chloride and formaldehyde. The final pH value amounts to 5.0. A very light, fully tanned leather of a pliable and mellow feel is thus obtained. The increase in thickness amounts to about despite the much higher pH value, as compared with the leather retanned only with the lignin sulphonic acid product.

Example 5 100' parts of goat pelt are pretanned in the usual manner with 10 parts of a commercial synthetic pretanning agent. After pretanning the pH value amounts to 4.5. Retanning is carried out as described in Example 3 in a fresh solution (100 parts) of parts of a tanning agent obtained from brenzoel, ethanolamine and formaldehyde as described in Example 24 of copending application No. 241,693, in parts of water; the final pH value is 5.4. The tanning agent is prepared as follows: A solution of grams of brenzoel, 50 cc. water, 31 grams of ethanol amine and 40-41 cc. of concentrated hydrochloric acid, is mixed with 100 cc. of formaldehyde solution 30% and heated in a boiling water bath for 2 hours. The condensation product obtained is soluble in water in all proportions. A yellow-brown, fully tanned, somewhat firm leather of good properties is obtained.

By carrying out retanning with 45 parts of the tanning agent employed in Example 1, which is obtained from resorcinol, ammonium chloride and formaldehyde (final pH value 5.0), a very light, reddish-yellow, compact and fully tanned leather of a pliable feel is obtained.

Example 6 100 parts of goat pelt pickled with 100 parts of water, 10 parts of sodium chloride and 1 part of hydrochloric acid are pretanned in 100 parts of water with 30 parts of the tanning agent employed in Example 1 (which is obtained from resorcinol, ammonium chloride and formaldehyde), dissolved in 50 parts of water in the following manner.

The tanning agent solution is added in 4 portions in intervals of hour each and the hides are moved about therein overnight. After squeezing for A2 hour the leather is retanned in a fresh solution with 4 parts of a commercial 4/ 12 basic chromium sulphate of a chromium oxide content of 26%. Two hours after the last addition of the chrome tanning agent solution it is neutralized with 0.3 part of sodium carbonate dissolved in water (1 in two portions. During addition of the sodium carbonate the tanning agent solution initially becomes turbid but becomes clear again after a short time. The hides are moved about in the neutralized solution for a further 2 hours and then taken out. The final pH value of the neutralized solution is 4.8. After stuffing and drying a grey-beige coloured, fully tanned leather of a fine, pliable and mellow feel is obtained.

ExampLe 7 100 parts of goat pelt are pretanned in the usual manner with 4 parts of a commercial chrome tannin (containing 26% of chromium oxide) and neutralized with 0.3 part of sodium carbonate dissolved in water (1:20). The pretanned leather is taken out of the solution and stored overnight and then retanned in a fresh solution containing 30 parts of the tanning employed in Example 6, which is obtained from resorcinol, ammonium chloride and formaldehyde, as follows.

The tanning agent dissolved in 60 parts of water is added in 4 portions in intervals of hour each to the leather placed in 75 parts of solution. The leather is milled in the solution overnight and the solution is neur tralized the next morning to a pH value of 5.5 by careful addition of a 5% soda solution. A light grey, fully tanned leather is obtained. A substantially softer leather is obtained by omitting the neutralization step after chrome pretanning but adding the synthetic tanning agent to the exhausted chrome bath as described above and neutralizing to a pH value of 5.0 with soda solution (1:20) after moving overnight.

Example 8 100 parts of East Indian cabretta are thoroughly milled and retanned thereafter with a solution of parts of a tanning agent employed in Example 2, which is obtained from brenzoel, aniline hydrochloride and formaldehyde, in 40 parts of water as described in Example 3. The final pH value is 4.2. A yellow-brown leather of good properties is obtained which shows an increase in thickness of about as compared with the leather which was not retanned. By neutralizing the tanning solution towards the end of the tanning process to a pH value of 5.6- 5.8 by careful addition of ammonia solution a more fully tanned leather is obtained.

Example 9 parts of the sammed weight of bottom leather (neats leather) which has been tanned with a mixture of vegetable and synthetic tanning agents are milled in a current of air with 25 parts of the 20% solution of the amphoteric tanning agent employed in Example 1, which is obtained from resorcinol, ammonium chloride and formaldehyde. The leather is subsequently milled with a concentrated solution of 0.5-1% of sodium acetate (calculated on the sammed weight). The leather showing a pH value of 3.8-4 after drumming in the tanning agent, is neutralized by the aftertreatment to a pH value of about 4.3-4.5.

A leather of good colour, uniform properties and substantially decreased contents of substances which are capable of being washed out is obtained. Despite the addition of the tanning agent the decrease of the substances capable of being washed out amounts to 15-20% as compared with the substances contained in the original nontreated leather.

We claim:

1. A tanning process which comprises treating at least one member of the group consisting of animal hides, skins and pre-tanned leather with an aqueous solution of a water-soluble, acid-reacting salt of an amphoteric condensation product obtained by treating at temperatures up to about 100 C., mixtures containing polyhydric phenols having at least one active nuclear hydrogen, nitrogencontaining compounds of the general formula wherein R1 stands for a radical selected from the group consisting of hydrogen, alkyl, cycloalkyl, aryl and aralkyl, and R2 and R3 stand for radicals selected from the group consisting of hydrogen and alkyl, and saturated lower aliphatic aldehydes, and towards the end of the tanning neutralizing, the amine salt groups of the tanning solution by adding an acid binding compound selected from the group consisting of alkali metal hydroxides, alkali metal carbonates and ammonia.

2.'A tanning process as claimed in claim 1, wherein the treatment is effected by adding the tanning solution in pre-determined sequential portions.

3. A tanning process as claimed in claim 1, wherein the acid-reacting salt is obtained directly during the formation of the amphoteric condensation product, by employing the nitrogen-containing compound in the form of its acid-addition salt.

4. A tanning process as claimed in claim 1, wherein the acid-reacting salt is obtained subsequent to the formation of the amphoteric condensation product by treatment of said condensation product with an acid.

References Cited in the file of this patent UNITED STATES PATENTS 1,841,840 Muller Jan. 19, 1932 1,912,593 Pollak June 6, 1933 1,919,756 Somerville July 25, 1933 1,985,439 Blackadder Dec. 25, 1934 2,012,928 Hassler Aug. 27, 1935 2,036,916 Bruson Apr. 7, 1936 2,129,553 Russell et al. Sept. 6, 1938 2,191,943 Russell Feb. 27, 1940 FOREIGN PATENTS 200,262 Great Britain July 12, 1923 OTHER REFERENCES Windus, I. A. L. C. A., pp. 93 to 96, vol. 47, No. 2, February 1952. 

1. A TANNING PROCESS WHICH COMPRISES TREATING AT LEAST ONE MEMBER OF THE GROUP CONSISTING OF ANIMAL HIDES, SKINS AND PRE-TANNED LEATHER WITH AN AQUEOUS SOLUTION OF A WATER-SOLUBLE, ACID-REACTING SALT OF AN AMPHOTERIC CONDENSATION PRODUCT OBTAINED BY TREATING AT TEMPERATURES UP TO ABOUT 100* C., MIXTURES CONTAINING POLYHYDRIC PHENOLS HAVING AT LEAST ONE ACTIVE NUCLEAR HYDROGEN, NITROGENCONTAINING COMPOUNDS OF THE GENERAL FORMULA 